Conventionally, there is provided a semiconductor relay often called a MOSFET output photocoupler or an optical MOSFET (see, e.g., Patent document 1).
FIGS. 11 to 14 illustrate an example of a semiconductor relay 1. The semiconductor relay 1 includes two input terminal portions 51 and two output terminal portions 61. The semiconductor relay 1 further includes a light emitting element 2 connected between the input terminal portions 51, two MOSFETs 3a and 3b serially connected to each other in between the output terminal portions 61 and a light-receiving drive element 4 for switching the MOSFETs 3a and 3b on and off depending on the presence and absence of the light emitted by the light emitting element 2.
The light emitting element 2 has a pair of terminals electrically connected to the input terminal portions 51, one to each input terminal portion. The light emitting element 2 emits light as a specified electric power (e.g., a DC power having a specified voltage) is applied between the terminals. As the light emitting element 2, it is possible to use, e.g., a light emitting diode.
The light-receiving drive element 4 includes a light receiving element 4a configured to receive the light from the light emitting element 2 to generate an electric power and formed of, e.g., an array of photodiodes, and a drive circuit 4b configured to switch on the MOSFETs 3a and 3b with the electric power supplied from the light receiving element 4a during the time period when the light receiving element 4a generates the electric power and to switch off the MOSFETs 3a and 3b during the time period when the light receiving element 4a does not generate the electric power. The light receiving element 4a and the drive circuit 4b are integrated into a single chip. The light-receiving drive element 4 is well-known in the art and therefore will not be described in detail.
Each of the MOSFETs 3a and 3b is of an N-channel type. The source electrodes 31 of the respective MOSFETs 3a and 3b are electrically connected to each other. The gate electrodes 32 and the source electrodes 31 of the respective MOSFETs 3a and 3b are electrically connected to the drive circuit 4b of the light-receiving drive element 4. The drain electrodes 33 of the respective MOSFETs 3a and 3b are electrically connected to the output terminal portions 61, one to each output terminal portion. In other words, the parasitic diodes of the respective MOSFETs 3a and 3b are reversely arranged with respect to each other. Thus, the electric connection (conduction) between the output terminal portions 61 connected to the respective ends of the serial circuits of the two MOSFETs 3a and 3b are switched on and off depending on the on/off condition of the MOSFETs 3a and 3b regardless of the direction of the voltage applied to between the output terminal portions 61.
The operation of the semiconductor relay 1 will now be described. During the time period when a specified electric power is not inputted to between the input terminal portions with the light emitting element 2 turned off, the respective MOSFETs 3a and 3b are kept switched off. Thus, the electric connection between the output terminal portions 61 is switched off.
If a specified electric power is inputted to between the input terminal portions 51 and if the light emitting element 2 is turned on, the light-receiving drive element 4 switches on the MOSFETs 3a and 3b, whereby the electric connection between the output terminal portions 61 is switched on.
If the light emitting element 2 is turned off, the light-receiving drive element 4 switches off the MOSFETs 3a and 3b, whereby the electric connection between the output terminal portions 61 is switched off again.
In this regard, the semiconductor relay 1 includes an encapsulating resin 10 for encapsulating the light emitting element 2, the MOSFETs 3a and 3b and the light-receiving drive element 4. At least the portion of the encapsulating resin 10 interposed between the light emitting element 2 and the light receiving element 4a of the light-receiving drive element 4 is formed of a synthetic resin capable of transmitting the light emitted from the light emitting element 2.
The semiconductor relay 1 further includes, in the encapsulating resin 10, two input conductor plates 5 made of an electrically conductive material and having connection portions 52 electrically connected to the respective terminals of the light emitting element 2. One terminal of the light emitting element 2 is surface mounted on the connection portion 52 of one of the input conductor plates 5. The other terminal of the light emitting element 2 is electrically connected to the connection portion 52 of the other input conductor plate 5 by wire bonding. The respective input conductor plates 5 have input terminal portions 51 protruding outside the encapsulating resin 10. The respective input conductor plates 5 can be formed by, e.g., subjecting a metal plate to punching and bending.
The semiconductor relay 1 further includes, in the encapsulating resin (package) 10, two output conductor plates 6 made of an electrically conductive material and having mount portions 62 on which the drain electrodes 33 of the MOSFETs 3a and 3b are respectively surface mounted and a central conductor plate 7 made of an electrically conductive material into a flat shape. The light-receiving drive element 4 is surface mounted on the central conductor plate 7 while the central conductor plate 7 is encapsulated with the encapsulating resin 10. Similar to the input conductor plates 5, the output conductor plates 6 and the central conductor plate 7 can be formed by subjecting a metal plate to punching and bending.
The source electrodes 31 of the respective MOSFETs 3a and 3b are electrically connected to each other by wire bonding. The gate electrodes 32 are electrically connected to the light-receiving drive element 4 by wire bonding. Further, the source electrode 31 of the MOSFET 3a and the light-receiving drive element 4 are electrically connected to the central conductor plate 7 by wire bonding. Thus, the source electrodes 31 of the respective MOSFETs 3a and 3b are electrically connected to the light-receiving drive element through the central conductor plate 7. Further, the respective output conductor plates 6 have output terminal portions 61 protruding outside the encapsulating resin 10.
As shown in FIGS. 13 and 14, the input terminal portions 51 and the output terminal portions 61 are respectively surface mounted on the electrically conductive patterns P1 and P2 formed on the mount surface (the upper surface in FIG. 14) of a common printed circuit board P.
In the conventional case, the thickness direction of the respective mount portions 62 and the central conductor plate 7 coincides with the thickness direction of the printed circuit board P.    Patent document 1: Japanese Patent Application Publication No. 2003-8050
In the above case, when a high-frequency signal is transmitted between the output terminal portions 61, the central conductor plate 7 connected to the source electrodes 31 of the respective MOSFETs 3a and 3b functions as a so-called stub circuit. Thus, the high-frequency signal flows into the central conductor plate 7.
Further, in the printed circuit board P, it is sometimes the case that a conductive ground pattern P3 having the same electric potential as the ground is formed on the entirety of the non-mount surface (the lower surface in FIG. 14) in order to shield, e.g., an electromagnetic noise. In this case, a parasitic capacitance Cp is generated between the mount portions 62 of the respective output conductor plates 6 and the ground pattern P3 opposite to each other and between the central conductor plate 7 and the ground pattern P3 opposite to each other. If such is the case, the high-frequency signal flows out from the mount portions 62 of the respective output conductor plates 6 or the central conductor plate 7 through the parasitic capacitance Cp. This leads to an increase in insertion loss.